The demands set by the paper making industry for the tension of wires in the form of paper machine wires have risen sharply within the sheet-forming zone since introduction of so-called gap former technology. Consequently, both better dimensional stability in the transverse and longitudinal directions of the fabric in the event of high initial moisture removal accompanied by high retention and a low-marking wire surface structure are currently required of look-through wires. At the same time, the paper machine wire is expected to have an ever longer service life despite the constantly increasing production speeds.
The aggregate of the pertinent requirements contains a large number of contradictory quality demands made of the look-through wire. In the course of time, solutions have been proposed for many of the problems areas previously mentioned.
AT 28 339 B discloses a composite fabric for a paper machine wire with an upper and a lower sheet, that is, with two warp sheets. The two warp sheets optionally have weft or warp threads, with one of the warp sheets forming an upper side and another warp sheet forming the lower side of the fabric. Weft threads are woven at least into the warp sheet forming the upper side. Weft threads in the form of associated pairs both continuously overlap and underlap the individual warp threads of the two fabric sheets. The disclosed composite fabric results in an especially firm bond, but is characterized by low flexibility when applied and is subject to extensive wear.
DE 32 29 307 A1 discloses weaving of three superimposed weft sheets into a densely filled warp to produce high bending strength in the transverse direction. The longitudinal direction remains flexible to the same extent as known for double-sheet wires, described in U.S. Pat. Nos. 4,071,050, 4,041,989, and 4,112,982.
A second warp sheet is to be bound into the fabric to increase the stability of the fabric in the longitudinal direction as well. Conventional original forms of the relevant composite fabric are described in DE 29 17 694 A1 and in EP 0 141 791 A1. In DE 29 17 694 A1, two complete single-sheet fabrics are bound by a binding weft system. In EP 0 141 791 A1, a single-sheet fabric is bound to a two-sheet wire by binding threads.
Since additional binding threads disrupt the uniform surface structure of the fabric, the concept of separate binding threads in composite fabrics was abandoned, and the binding threads were incorporated into the surface structure of the fabric. Examples of the latter process are in U.S. Pat. No. 5,152,326, EP 0 069 101 A1, and PCT/WO 99/06630. In this new type of composite fabric, the conventional binding thread has been replaced by a pair of binding threads. This pair of binding threads exerts a supplementary effect in the fabric, in that, in the areas in which the first of these two binding threads is bound into the upper sheet of the fabric in the form of a fiber-bearing thread, the second binding thread binds to the warp of the lower fabric. The two threads thus intersect in the interior of the fabric structure and then exchange the functions they perform relative to fiber support and binding of the fabric sheets. The resulting advantage is represented by the substantially more rigid fabric binding of upper sheet and lower sheet. In the case of these fabrics, the situation is such that the diameter to be selected for the pair of binding threads is just as large as the diameter of the other transverse threads in the upper fabric.
Almost all of the composite fabrics of this type, whether the binding is effected by the conventional binding weft (DE 29 17 694 A1) or by a pair of binding threads (U.S. Pat. No. 5,152,326), have in common that the upper side of the fabric is preferably designed as a basket weave. A weave of this type causes the least marking in the paper and has the highest fiber support index. In the basket weave, each weft wire is bent around each warp wire. As a result, a large number of uniform points of application is created for the paper fibers. From the viewpoint of stability, however, the basket weave is the most unstable form of weave for a paper machine wire. Every bend in the weft around the warp may also be regarded as an impressed hinge or link point, so that precisely the basket-weave fabric possesses the highest turning and bending capability. In the case of composite fabrics, the fabric structure is stabilized in that the lower fabric is in the form of at least a four-shank fabric, preferably as a five-shank and in rare cases a six-shank fabric. These stable weaves, in conjunction with the basket weave on the upper side, also result in a total fabric characterized by acceptable performance and use data.
The constantly increasing production speeds in the paper machine industry and the associated increasing loads placed on paper machine coverings, however, necessitate continuous improvement in the stability requirements.
U.S. Pat. Nos. 5,881,764 and 5,967,195 disclose generic papermaking wires. Their thread-bearing bridge is in the form of a support weft thread, supporting a maximum of four warp threads of the upper fabric and extending at the maximum along four warp threads of the lower fabric. The support situation for the thread-bearing bridge occurs only once within a repeat of an eight or ten shank binding. Otherwise, only two or three warp threads of the upper fabric are supported. The thread-supporting bridge also extends along only two or three warp threads of the lower fabric, which to this extent in turn support the thread-supporting bridge. Consequently, since the thread-bearing bridge formed is designed to float only for a brief period, this bridge also extends at least in part along an oblique path plane, impairing the behavior of the binding between the fabric sheets. Accordingly, the support weft threads also alternate rapidly between formation of the thread-bearing bridge and of the alternating encircling pattern for the upper fabric. In this way, a plurality of binding points of the respective support weft thread is achieved in the lower fabric, to achieve secure binding of upper and lower fabric sheets. Because of the plurality of binding points thereby achieved, the fabric binding is relatively rigid, something which has an unfavorable effect in the reversal areas for the papermaking wire. In addition, friction occurs between the fabric sheets, and is accompanied by correspondingly high wear of the composite fabric.